(a) Field of the Invention
The present invention relates to an organic light emitting diode (“OLED”) display and a manufacturing method thereof.
(b) Description of the Related Art
Recent trends toward lightweight and thin personal computers and televisions sets accordingly require lightweight and thin display devices, and flat panel displays satisfying such a requirement are being substituted for conventional cathode ray tubes (“CRTs”).
The flat panel displays include a liquid crystal display (“LCD”), a field emission display (“FED”), an organic light emitting diode (“OLED”) display, a plasma display panel (“PDP”), among others. Among the flat panel displays, the OLED display is the most promising because of its low power consumption, fast response time, wide viewing angle and high contrast ratio.
An OLED display is a self-emissive display device which includes two electrodes and an organic light emitting layer interposed therebetween. One of the two electrodes injects holes and the other electrode injects electrons into the light emitting layer. The injected electrons and holes are combined to form exitons, which emit light as discharge energy.
The OLED displays may be divided into a passive matrix OLED display and an active matrix OLED display by a driving method thereof.
The passive matrix type OLED display includes a plurality of anode lines, a plurality of cathode lines intersecting the anode lines, and a plurality of pixels, each including a light emission layer. The selection of one of the anode lines and one of the cathode lines cause light emission of a pixel located at the intersection of the selected signal lines. In contrast, the active matrix type OLED display includes a plurality of pixels, each including a switching transistor, a driving transistor, and a storage capacitor as well as an anode, a cathode and a light emission layer. The driving transistor receives a data voltage from the switching transistor and drives a current having a magnitude determined by the data voltage. The current from the driving transistor enters the light emission layer to cause light emission having an intensity based on the driving current.
In the active matrix type OLED display, characteristics of the switching thin film transistor (“TFT”) and those of the driving TFT may be different from each other to optimize the characteristics of the OLED. In more detail, the switching TFT may have a good on/off characteristic, and the driving TFT may have a high mobility and stability for supplying sufficient current for driving the OLED.
In the driving TFT, a polycrystalline semiconductor may be used for improving the field-effect mobility and stability of the driving TFT.
However, if the polycrystalline semiconductor is used in the switching TFT, then the on/off characteristic thereof may be deteriorated due to large leakage current of the polycrystalline semiconductor. Accordingly, the data voltage transported from the switching TFT to the driving TFT may be reduced to generate cross-talk.